1. Field of the Invention
The present invention relates to a wet type flue-gas desulfurization apparatus mainly used in a thermal power plant or the like. Particularly, it relates to a wet type flue-gas desulfurization apparatus (hereinafter referred to as desulfurization apparatus) provided with an absorption tower for eliminating sulfur oxide (SOx), mercury (Hg), etc. contained in gas to be treated, such as exhaust gas.
2. Description of the Background Art
In order to prevent air pollution, wet type limestone-gypsum process desulfurization apparatuses have been put into practice broadly as apparatuses for eliminating SOx and so on from exhaust gas discharged from combustion equipment such as boiler apparatuses installed in factories, steel mills, chemical plants, thermal power plants, etc.
FIG. 6 is a schematic configuration diagram of a boiler plant for thermal power generation, which uses coal as fuel.
As shown in FIG. 6, this type boiler plant for thermal power generation is chiefly constituted by a boiler apparatus 13, a denitration apparatus 14, an air preheater 15, an electrical precipitation apparatus 16, a desulfurization apparatus 3, etc.
The boiler apparatus 13 combusts coal 25 to generate exhaust gas. The denitration apparatus 14 breaks down nitrogen oxide (NOx) contained in the exhaust gas discharged from the boiler apparatus 13.
The temperature of the gas treated by the denitration apparatus 14 is adjusted to 160 to 200° C. by the air preheater 15, and ash dust is eliminated from the exhaust gas by the electrical precipitation apparatus 16. From the de-dusted exhaust gas, SOx and so on are eliminated by the desulfurization apparatus 3, and released into the atmosphere through a stack 29.
FIG. 5 is a schematic configuration diagram of a desulfurization apparatus 3 according to the background art.
As shown in FIG. 5, the desulfurization apparatus 3 is chiefly constituted by an absorbent spray nozzle 4, an absorbent circulation pump 5, a mist eliminator 8, an oxidation gas supply portion 9, an agitator 10, etc.
An absorbent 6 composed of limestone slurry and sprayed from the absorbent spray nozzle 4 is brought into gas-liquid contact with exhaust gas 1. Thus, SOx such as SO2 is absorbed and eliminated together with ash dust or acid gas such as hydrogen chloride (HCl), hydrogen fluoride (HF), etc. in the exhaust gas 1.
The absorbed SO2 becomes sulfurous acid. When the concentration of sulfurous acid is high, the efficiency in absorbing SO2 is lowered. Therefore, oxidation gas 27 such as the air is supplied from the oxidation gas supply portion 9 so as to oxidize the sulfurous acid into gypsum. Thus, the performance to absorb SO2 is resumed. The absorbent 6 containing gypsum is supplied to a not-shown dehydrator and extracted as gypsum from the desulfurization apparatus 3.
In FIG. 5, the reference numeral 2 represents exhaust gas in the outlet of the desulfurization apparatus; 11, an absorbent reservoir portion; and 26, a desulfurization absorption portion.
For example, JP-A-2010-269277, JP-A-5-337331, JP-A-2005-125261, JP-A-2006-136856, etc. have disclosed this type flue-gas treatment apparatus.
When Hg contained in flue gas of coal is eliminated by the desulfurization apparatus according to the background art, the Hg concentration in the absorbent 6 increases gradually. In the background art, there is no consideration about the point that absorbed Hg may be moved to the gypsum side. There is a problem that it is difficult to use recovered gypsum because the quality of gypsum deteriorates due to increase in Hg concentration on the gypsum side.
In addition, in the background art, there is no consideration about the point that Hg may be released again to the exhaust gas. There is a problem that the exhaust gas in the outlet cannot keep the regulation value of Hg.
Further, there is no consideration about the point that make-up water may be required due to the evaporation of the absorbent when the exhaust gas in the inlet of the desulfurization apparatus is cooled. In a region where industrial water is limited, fresh water generated from a desalination apparatus is used as the make-up water. There is a problem that the cost increases.
Examples of desalination methods include a multi-stage flash method, a membrane separation method, etc. In any method, concentrated water containing Cl ions of about 50,000 to 60,000 ppm is discharged in addition to desalted water with a lowered Cl-ion concentration. However, the Cl-ion concentrated water is often discharged directly to the sea because there is no place where the Cl-ion concentrated water can be used. The efficiency of the system as a whole can be further increased if the concentrated water can be used to save the water consumption of the desulfurization apparatus.